Collagen fibrillogenesis involves the stepwise formation of linear and lateral aggregates from collagen molecules. Characterization of these aggregates in vitro and in vivo is being pursued using laser light scattering and electron microscopy. These studies focus on identifying the chemical and physical forces that promote assembly. Laser light scattering studies suggest that collagen assembly occurs by the formation of linear trimers, lateral aggregates of linear trimers, and linear and lateral aggregation of twisted units. The exact mechanism beyond the formation of twisted units is currently being investigated. Electron microscopy of SLS derived from collagen linear aggregates show the presence of two major types of associations. The associations between collagen molecules in vitro and in vivo are being studied.